IODIDE AS THE MEDIATOR FOR THE REDUCTIVE REACTIONS OF PEROXIDASES

Lignin peroxidase H2 (LiPH2) from the white rot fungus Phanerochaete c hrysosporium catalyzed the reduction of cytochrome c, nitro blue tetra zolium, ferric iron, molecular oxygen, and triiodide in a reaction mix ture containing LiPH2, H2O2, EDTA, and iodide. Activity followed first order kinetics with respect to EDTA concentration. The reductive acti vity observed with LiPH2 using iodide as the mediator was comparable t o that obtained using a variety of other free radical mediators such a s veratryl alcohol, 1,4-dimethoxybenzene, and 1,2,3- and 1,2,4-trimeth oxybenzene. EDTA-derived radicals were detected by ESR spin trapping u pon incubation of LiPH2 with H2O2, iodide, and EDTA. Reduction activit y was also observed using other peroxidases such as lactoperoxidase, h orseradish peroxidase, and myeloperoxidase. For the reduction activity of LiPH2, it is proposed that the oxidation of EDTA is mediated by th e iodide radical, and the reduction of various electron acceptors is m ediated by EDTA radicals. The inhibition of reduction activity at high er concentrations of iodide might be due to the combination of iodide radicals to form I2 which forms a stable triiodide complex by reacting with excess iodide.